1. Technical Field
The invention relates to the field of photomultipliers, and more particularly, to an improved photocathode and a method for making the same.
2. Background Art
Photomultipliers and image intensifier devices employ a photocathode for conversion of photons to electrons. Microchannel plate image intensifiers are currently manufactured in two types that are commonly referred to as generation II (Gen II) and generation III (Gen III) type image tubes. The primary difference between these two types of image intensifiers lies in the type of photocathode employed.
Generation II image intensifier tubes have a polycrystalline multi-alkali photocathode, while generation III image intensifier tubes generally have a p-doped gallium arsenide (GaAs) photocathode that has been activated to negative electron affinity (NEA) by the adsorption of cesium and oxygen on the surface.
Existing photocathodes have several disadvantages. Generation III photocathodes are generally made using expensive processes such as metal/organic/chemical/vapor deposition (MOCVD) or molecular beam epitaxy (MBE). Compared to the prior techniques, the transparent matched substrate used in the present invention likely provides a cost advantage. Such production process is expensive and wasteful.
Additionally, the substrate and several of the subsequent growth layers of Gen III photocathodes must ultimately be wasted by being etched away in order to produce the actual Gen III photocathode. The Gen III photocathode must, by a separate process, also be attached to a window suitable for the wavelengths of interest.
Alkali antimonides have been the workhorses for photocathodes in photomultipliers and more recently GEN 2 image intensifiers starting with the discovery of Cs3Sb as a photoemitter in 1936. Since then, there have evolved a variety of materials, all polycrystalline small gap semiconductors, containing the alkali metals but all in the form M3Sb where M is either a single alkali or alkali alloy. The photoemissivity of members of this family are second only to the negative electron affinity (NEA) GaAs (GEN 3) photocathode.
Delft University of Technology and Dr. A. R. H. F. Ettema at that institution have shown only growth of K3Sb:Cs on a vanadium substrate with no characterization to indicate epitaxy [Appl. Surf. Sci. 175–6, 101 (2001)]. Vanadium has a lattice constant of 3 Å. The lattice mismatch is too extreme for epitaxy to be likely.
While the above cited references introduce and disclose a number of noteworthy advances and technological improvements within the art, none completely fulfills the specific objectives achieved by this invention.